8 replies to this topic

[quilty]

Here it is.

Just multiply the ep's fl with its AFOV and divide by 1.000. This yields the angular FOV size for a scope with 1 m fl.

Example: ES 62° 26 mm yields 1.612, say 1.61° in an 1 m fl scope. A 40 mm Plössl with 40° AFOV yields 1.600 say 1.6° so no larger FOV here.
A 24 mm ES 68° is no help neither, 1.62°
Division by 1k is a no brainer and the multiplication can be done without any help, too.

[Starman1]

Here it is.

Just multiply the ep's fl with its AFOV and divide by 1.000. This yields the angular FOV size for a scope with 1 m fl.

Example: ES 62° 26 mm yields 1.612, say 1.61° in an 1 m fl scope. A 40 mm Plössl with 40° AFOV yields 1.600 say 1.6° so no larger FOV here.
A 24 mm ES 68° is no help neither, 1.62°
Division by 1k is a no brainer and the multiplication can be done without any help, too.

Alas, it tends to overstate some TFOVs and be close on others.
This is because true fields are directly related to field stops. For example, field stops in 24mm 1.25" eyepieces range from 26.9 to 28.0mm and the 28.0mm eyepiece has the same apparent field as the 26.9mm field stop, yet a true field 4.1% wider.

A 40mm 43° Tele Vue Plossl has exactly the same true field as their 24mm 68°, yet multiplying the numbers suggests the 40mm is 5.4% wider.

Distortion changes the amount of true field in an eyepiece, and disortio differs by sometimes large amounts betwee eyepieces, making the simple magnifier possibly useful if comparing eyepieces far apart in focal length, but not very useful for seeking a larger true field. For that, look at the field stops.

Edited by Starman1, 16 April 2025 - 04:26 PM.

[Jon Isaacs]

Here it is.

Just multiply the ep's fl with its AFOV and divide by 1.000. This yields the angular FOV size for a scope with 1 m fl.

Example: ES 62° 26 mm yields 1.612, say 1.61° in an 1 m fl scope. A 40 mm Plössl with 40° AFOV yields 1.600 say 1.6° so no larger FOV here.
A 24 mm ES 68° is no help neither, 1.62°
Division by 1k is a no brainer and the multiplication can be done without any help, too.

 

If you just divide by the telescope's focal length then you have 

 

TFoV = AFoV / Mag  

 

One is likely to need a calculator to do AFoV x Focal length so dividing by the scopes focal length provides an estimate of the actual TFoV, keeping in mind, Don's comments.

 

Jon

[Sandy Swede]

All well and good if you have a one meter scope, if not, you have to divide by its FL, correct?. So, using quilty's formula for my Panoptic 24mm with an AFoV of 68^o and my 9.25" EdgeHD, then 24 x 68 = 1.632 / 2,350 = .69^o TFoV.  Using the formula with which I am familiar: TFoV = (Eyepiece Field Stop Diameter / Scope Focal Length) x 57.3, then my result is 27 / 2,350) x 57.3 = .66^o. Is this appropriate? Also, please check my math. Since the purpose of quilty's thread is to have a quick approximation, I have to state that a difference in TFoV of .03^o is "no biggie." Now I have killed all of the brain cells quilty tried to save me.

 

Bonus round: Why do we multiply by 57.3 in the above formula?

[KBHornblower]

Bonus round:  That is to convert from radians to degrees.

[Sandy Swede]

Bonus round:  That is to convert from radians to degrees.

And the winner is KBHornblower.  Pi Radians converted to degrees = 180^o (Pi Radians) / 3.1416 (Pi) = 57.3

[Jon Isaacs]

And the winner is KBHornblower.  Pi Radians converted to degrees = 180^o (Pi Radians) / 3.1416 (Pi) = 57.3

 

57.3°/radian

[Jon Isaacs]

I should add that 180 degrees / Pi is approximately 57.295779513082320876798154814105 deg/rad but that the error in using 57.3 deg/rad is less than 0.01%, plenty good enough since none of the measurements are made to that level of accuracy.

 

Jon

[NinePlanets]

You can also simply record the time a roughly ~0 declination star centrally transits the AFOV and divide to get the true FOV.

 

ie: At 0 degrees declination 1 hour of RA = 15 degrees

20 min of RA = 5 degrees

4 min of RA = 1 degree

etc. ...

 

But is much faster to simply measure the field stop of the eyepiece (if you can get to it).

Edited by NinePlanets, 19 April 2025 - 09:43 AM.